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Abstract: We investigated the existence of the “winner effect” (winning an aggressive encounter following previous victories) and an associated rise in testosterone (T) in the white-footed mouse (Peromyscus leucopus) which generally display low levels of aggression and territoriality. We compared the effect of previously winning three, two, one, or zero resident–intruder encounters on the likelihood of winning a subsequent aggressive encounter. Although 50% of males were removed during training because of peaceful encounters, the winner effect was weak and not significant. We hypothesize that territoriality/aggression may be associated with the strength of the winner effect and discuss whether the slight winner effect exhibited by P. leucopus may become significant when population densities increase and males become more territorial. There was also no associated change in T with winning; however, corticosterone (Cort) changed with experience as winners had low Cort levels compared to losers and controls. Furthermore, low Cort levels in winners were associated with quicker attack latencies. These results contrast with findings of a significant winner effect and increase in T in males of the highly territorial and aggressive California mouse (Peromyscus californicus) using an identical methodology. California mice also attacked their opponents at more caudal regions of the body compared to white-footed mice that attacked their opponents at more rostral regions of the body, possibly related to different levels/types of aggression expressed by the two species. [Copyright &y& Elsevier]

Abstract: The ‘winner effect’ has been studied in a variety of species, but only rarely in mammals. We compared effects of winning three, two, one, or zero resident–intruder encounters on the likelihood of winning a subsequent aggressive encounter in the California mouse (Peromyscus californicus). During the training phase, we ensured that resident males won all encounters by staging contests with mildly sedated, smaller intruders. During the test phase, the resident male encountered an unfamiliar, more evenly matched intruder that had experience winning an encounter and was larger than the resident. Testosterone (T) plasma levels significantly increased after the final test when they had experienced two prior winning encounters, and the probability of winning a future encounter increased significantly after three prior wins independent of intrinsic fighting ability. We hypothesize a ‘winner–challenge’ effect in which increased T levels serve to reinforce the winner effect in male California mice. [Copyright &y& Elsevier]

Abstract: The functions of rapid increases in testosterone seem paradoxical because they can occur in response to different social contexts, such as male–male aggressive encounters and male–female sexual encounters. This suggests that context may impact the functional consequences of changes in testosterone, whether transient or long term. Many studies, including those with California mice (Peromyscus californicus), have addressed these issues using manipulations and species comparisons, but many areas remain to be investigated. We report a study here that suggests transient increases in testosterone after social competition influence future competitive behavior, but social experience alone may also be critical in determining future behavior. In other rodents, a comparable testosterone surge occurs in response to sexual stimulation, but the function is not entirely understood. In addition to competitive and sexual behavior, testosterone impacts other systems instrumental to social behaviors, including paternal behavior and degree of monogamy. Thus, mechanisms regulated by testosterone, such as the vasopressin and aromatase systems, may also be influenced by rapid surges of testosterone in aggressive or sexual contexts. We discuss how the functions of testosterone may overlap in some contexts. [Copyright &y& Elsevier]